Display devices include, for example, liquid crystal display devices (LCD devices), organic electro-luminescence (OEL) display devices, plasma display panels (PDPs) and the like. A liquid crystal display device includes, for example, a liquid crystal panel including a pair of substrates which are combined together to face each other, and a backlight unit located so as to face a rear surface of the liquid crystal panel. The liquid crystal panel includes a liquid crystal layer between the pair of substrates, and can control the light transmissivity by controlling the voltage to be applied between the pair of substrates. The liquid crystal display device displays an image in a display region of the liquid crystal panel by irradiating the display region with light emitted from an illumination section located in the backlight unit while controlling the form of the liquid crystal layer of the liquid crystal panel. When the display region of such a liquid crystal display device is irradiated with external light, the external light is reflected by a surface of the liquid crystal panel, which may decrease the contrast of the displayed image. Herein, the term “external light” means light emitted by an other element than a display device (encompassing a liquid crystal display device). In the case where, for example, a liquid crystal display device is located in a room, light from an illumination device located in the room and light from outside the room are both “external light”.
Patent Document 1 discloses a method for controlling the backlight unit. According to this method, a plurality of optical sensors are attached to a plurality of different positions in a peripheral portion on a front surface side (display surface side) of a liquid crystal display device, and light receiving information obtained by the optical sensors is subjected to comparative computation. Based on the comparative computation result, the backlight unit is controlled. As can be seen, according to the method disclosed in this publication, the contrast of the displayed image is adjusted based on the brightness of external light in the peripheral portion of the display region.
With a liquid crystal display device disclosed in Patent Document 2, a plurality of optical sensors are located in the display region to sense the distribution of external light in the display region. This liquid crystal display device controls the voltage to be applied to the liquid crystal layer of the liquid crystal panel based on the distribution of the external light in the display region, and adjusts the light transmittance of the liquid crystal layer area by area. As can be seen, the liquid crystal display device disclosed in this publication adjusts the contrast of the displayed image by adjusting the light transmittance of the liquid crystal layer.
Although not disclosing a method for adjusting the contrast of a displayed image, Patent Document 3 discloses a display control system including a management section for managing the display region. The management section of this display control system manages an unoccupied part of the image screen. When, for example, one image is displayed on the display region, the management section performs processing such as enlargement/reduction, movement or the like on another image, such that the another image is displayed in the unoccupied part of the display region where no image is displayed. In this manner, the display control system displays a plurality of images on one image screen at the same time.